I’m in big trouble, and I need your help. Last week, my old friend, Lillian, who was recovering from a breakup, came to stay with me.

We were lying on my bed just chatting, when Lillian discovered my vibrator under my pillow. She never had used one before, and asked me about it. I explained how it worked, but she didn’t seem to get it.

Somehow, Lillian took her pants off, and used the vibrator, while I, in the spirit of helping a downtrodden and sexually-deprived friend, instructed her. Ultimately, she had an orgasm.

My girlfriend was not home at the time. When she got back, she asked me why the vibrator was out. I didn’t want to lie, so I told her. She was furious, and told me I had cheated on her.

Ms. Behavior, it never occurred to me that this was really sex. Lillian and I didn’t kiss or touch each other in any way. I will admit that I got a little turned on watching her, but my initial motivation was helping a friend, not my own sexual desire.

So, did I really cheat on my girlfriend? Do you have any guidelines for how to tell whether something counts as sex?

—Buzz

Dear Buzz:

While you may not have intended to cheat on your girlfriend, encouraging someone through an orgasm probably counts as infidelity (unless you’re a certified sex therapist or Ms. Behavior).

Though you are clearly a helpful and well-meaning person, giving remedial vibrator lessons to mechanically-disadvantaged lesbians will not bode well for your relationship with your girlfriend.

Lesbians often feign confusion about which acts count as actual sex. This vagueness occurs partly because heterosexist society prefers to define sex as penis-vagina penetration, which leaves lesbians totally out of the picture.

But lesbians also remain confused because vagueness about which acts constitute sex is titillating. Just ask any lesbian who has managed to reach orgasm with her clothing on.

So, let Ms. Behavior clear it up for you and millions of lesbians throughout the world: Sex is any act that potentially could result in an orgasm for either or any participant.

This means that it doesn’t matter whether you’re naked or fully-dressed; whether your lover minds; or whether you initially intended to have sex.

Bear in mind, though, that the definition of sex and the definition of fidelity within a particular relationship are not always the same thing.

While activities like phone sex or side-by-side masturbation unquestionably are actually sex, some partners don’t mind, and others do.

While kissing isn’t sex, kissing someone outside the context of a monogamous relationship is often considered cheating (at least by lesbian girlfriends whose partners have kissed other women).

In any case, apologize to your girlfriend for your transgression, and hope that she can forgive your misguided enthusiasm.

While a hands-on approach probably serves you well in most areas of your life, next time a friend comes to you with intimate questions or troubles, try to have a more detached approach.

Ask yourself: “If this were a douche or a tampon, would I get right in there and help, or would I just offer my friend a pamphlet?”

Dear Ms. Behavior:

I’m a 48-year-old man who recently starting dating a 22-year-old. Brad is sweet and cute. He appears to like me, but seems to expect me to pay for things. So far, it has been small things.

Brad suggested going out for pizza last night, and then, when it came time to pay, said he had no money. He also has no car, and expects me to drive him to school.

This feels weird to me. On the other hand, he’s very affectionate.

How can I tell if he is using me?

—Gary

Dear Gary:

People successfully “use” each other all the time, even when money is not part of the arrangement.

What’s wrong with Brad liking you because you pay for his dinner and make him feel safe, and you liking him because he’s young and pretty?

Ms. Behavior is fairly certain it’s not Brad’s scintillating conversation skills that keep you engaged. Anyway, you might find that a brief tryst with a cute young thing elevates your mood for a while, as long as you don’t expect too much.

Opposing expectations may cause a potential problem. If you want a lifelong partnership, while Brad wants lunch money and an alternative to the school bus, you’ll end up feeling used. In that case, you always can make him pack up his X-Box and his joystick, and go home.